1. Crust: This is the outermost, rocky layer, and what we walk on. It's relatively thin compared to the other layers.

2. Mantle: This is the thickest layer, made up of mostly solid rock but with a partially molten layer called the asthenosphere.

3. Outer Core: This layer is made of liquid iron and nickel. It's the source of Earth's magnetic field.

4. Inner Core: This is a solid ball of iron and nickel, despite being incredibly hot. The intense pressure at this depth keeps it solid.